Understanding the Difference Between Persist() and Merge() in JPA and Hibernate

In JPA and Hibernate, the persist() and merge() methods play critical roles in managing entities and handling data persistence. However, they have distinct differences in their behaviors.

Persist()

The persist() method, as defined by the JPA specification, operates as follows:

• If the entity is new and not managed, it becomes managed and will be saved to the database when the transaction is committed or flushed.
• If the entity is already managed or removed, the operation is ignored.
• If the entity is detached, an exception may be thrown, or the persist operation may fail during flush or commit time.
• The cascade operations are applied to any referenced entities with cascade=PERSIST or cascade=ALL annotations.

Merge()

On the other hand, the merge() method has a different set of semantics:

• If the entity is detached, its state is copied to a pre-existing managed entity with the same identity, or a new managed copy is created.
• If the entity is new, a new managed entity is created and its state is copied from the incoming entity.
• If the entity is removed, an exception is thrown.
• If the entity is managed, the operation is ignored, but cascade operations are still applied to referenced entities if annotated with cascade=MERGE or cascade=ALL.
• The merged entity references the managed entities for all relationships annotated with cascade=MERGE or cascade=ALL.
• If an entity is merged with a reference to an un-cascaded relationship, the association from the merged entity will return a reference to a managed object with the same identity.

When to Use Persist() or Merge()

Based on these differences, the following guidelines can help you choose the appropriate method:

• Use persist() when you want to create a new or update an existing entity that is not yet managed by the persistence context.
• Use merge() when you want to update an existing entity that is already managed but has been modified outside of the persistence context.